Heat transfer devices such as refrigerators, freezers, heat pumps and air conditioning systems are well known. In simple terms such devices typically operate via a cycle wherein a refrigerant of a suitable boiling point evaporates at low pressure taking heat from its surroundings, the vapor passes to a condenser where it condenses back to a liquid and gives off heat to its new surroundings, before returning to the evaporator completing the cycle. In addition to the mechanical parts, such as a compressor etc., specially suited materials are needed, including refrigerant, heat transfer materials, sealants to prevent loss of refrigerant and lubricants to allow for functioning of the movable parts. The lubricant in these devices must have good low temperature flow properties, be thermally stable, provide protection against wear of moving parts such as bearings under load, remove heat from the compressor and seal clearances to ensure efficient compression of gas from low to high pressure.
The refrigerant and the lubricating oil must circulate in the system without undergoing phase separation over a wide temperature range. In many cases there is a temperature below which the refrigerant and the lubricating oil undergo phase separation and there may also be a temperature above which they undergo phase separation. In general, a useful working fluid at a minimum will not undergo phase separation between 0 and 50° C., in many cases from below 0° C. to at least 60° C. That is, phase separation of refrigerant and lubricating oil in a useful working fluid in the low end will of the temperature range typically occurs below 0° C., e.g., temperatures below −10° C. or −20° C., and at the high end of the temperature range any phase separation should occur only at temperatures above 50° C., and in many devices, 60° C. or higher.
Excessive solubility of the refrigerant in the lubricant can also can also be problematic. For example, high concentrations of refrigerant in the lubricant can greatly reduce lubricant viscosity, leading to increased wear, shortened lifetime and lower performance of the device. Dissolved refrigerant in the lubricant can also cause foaming and bubbling of the lubricant mixture as it flows from one area of the compressor to another, e.g., from low to high temperature regions. Furthermore, refrigerant dissolved in the lubricant is essentially trapped and taken out of circulation, thereby reducing the cooling capacity of the system. As efficient functioning of a refrigeration lubricant requires not just proper lubricating properties and appropriate viscosities, but also appropriate compatibility with the refrigerant, changes in refrigerant frequently demand corresponding changes in lubricant.
U.S. Pat. No. 5,964,581 discloses polyol ester lubricants prepared by the condensation of pentaerythritol and monocarboxylic acids that are miscible with highly or fully fluorinated hydrocarbons such as 1,1,1,2-tetrafluoroethane, commonly known as HFC-134a, difluoromethane (HFC-32), trifluoromethane (HFC-23), pentafluoroethane (HFC-125), 1,1,2,2-tetrafluoroethane (HFC-134), 1,1,1-trifluoroethane (HFC-143a) and the like.
U.S. Pat. No. 6,444,626 discloses lubricants comprising poly(pentaerythritol) esters which contain mixtures of pentaerythritol esters, di-pentaerythritol esters, tri-pentaerythritol esters, tetra-pentaerythritol and higher oligomeric pentaerythritol esters, blended with a hindered polyol ester. U.S. Pat. No. 8,318,647 discloses refrigeration lubricants comprising select mixtures of carboxy esters of pentaerythritol, di-pentaerythritol and tri-pentaerythritol.
Concern about ozone depletion led to the replacement of chlorofluorocarbon refrigerants with alternate materials, such as highly or fully fluorinated hydrocarbons; and concern about climate change is leading to replacement, at least in part, of the presently used fluorinated alkanes with fluorinated olefins, e.g., hydrofluoro-olefin refrigerants (HFO), having a significantly lower global warming potential (GWP). For example, the fluorinated alkane refrigerant R-134a has a GWP of 1430; the fluorinated alkene R-1234ze (1,3,3,3-tetrafluoropropene) has a GWP of only 6 or less.
Ideally, hydrofluoro-olefin refrigerants would serve as “drop-in” replacements for the presently used fluorinated alkanes in refrigeration working fluids. However, it is not uncommon with refrigeration working fluids that changes in the refrigerant, or the conditions under which the working fluid is used, require changes in the lubricant. Even though many hydrofluoro-olefins are structurally similar to commercial saturated hydrofluorocarbons, simple replacement of saturated hydrofluorocarbons with hydrofluoro-olefins in the existing lubricant compositions has presented a number of challenges. For example, certain commercial hydrofluoro-olefins are much more miscible with polyester lubricant compositions than the saturated fluorocarbons that they would likely replace, which can cause unacceptable reduction in viscosity. As a result, mixtures of fluorinated olefins with fluorinated alkanes have been developed, as have miscibility additives for use with refrigerants such as hydrofluoro-olefins and various lubricants.
U.S. Published Patent Application No. 2013/0096218 discloses heat transfer compositions comprising mixtures of tetrafluoropropene, difluoromethane and tetrafluoroethane. U.S. Published Patent Application No. 2013/0092869 discloses a composition comprising polyol esters and a mixture of tetrafluoropropene, pentafluoropropene, and trifluoropropyne. U.S. Published Patent Application No. 2012/0011864 discloses the use of perfluoro-polyethers as additives to improve the characteristics of a wide variety of fluorinated refrigerants including hydrofluoro-olefins.
U.S. Pat. No. 8,603,354 discloses lubricating oil compositions for use with fluorine-containing refrigerants having a specific polar structure and a low global warming potential. Exemplified refrigerants include fluorinated ketones, fluorinated ethers and fluorinated alcohols.
U.S. Published Patent Application No. 2013/0099154 discloses a composition comprising polyol esters and tetrafluoropropene, however only ISO 68 lubricants are exemplified.
In some applications, refrigeration fluids containing lubricants having significantly higher viscosity are required, e.g., 150 centistokes or higher at 40° C., such as 160 to 350 centistokes at 40° C. and in some applications 190 cSt, 200 cSt, or higher and lubricants of ISO grade 220 to ISO 320 may be specified. High viscosity polyester lubricants have been formed from dipentaerythritol and higher molecular weight carboxylic acids, or alternatively, dicarboxylic acids are reacted with polyols to form complex esters, however difficulties arise when highly fluorinated hydrocarbons are used with these traditional higher viscosity polyester lubricants. It has also been found that these difficulties can become more pronounced when saturated hydrofluorocarbons are replaced with unsaturated fluorinated refrigerants, as seen with the enhanced miscibility of hydrofluoro-olefins referred to above.
Polyol esters based on pentaerythritol or di-pentaerythritol typically require branched acids (e.g., iso-C9 acid, i.e., 3,5,5-trimethylhexanoic acid) or very long-chain acids (e.g., n-C18 acids) to achieve high viscosity. However, POEs containing enough iso-C9 acid to obtain the desired high viscosity of the neat oil are often fully miscible with hydrofluoro-olef ins, such as R-1234ze. These highly branched POEs also exhibit a high affinity for the refrigerant (i.e., high solubility of the refrigerant in the oil), which causes a large decrease in viscosity of the working fluid and insufficient lubrication. On the other hand, POEs containing very long linear chain acids are immiscible with R-1234ze and are not viable candidates for refrigeration applications. Likewise, esters from di- or poly-carboxylic acids tend to be incompatible and/or unstable when used with hydrofluoro-olefins.
U.S. Pat. No. 6,774,093 discloses high viscosity polyol ester lubricants, comprising 30 to 45 wt % mono-pentaerythritol esters, 30 to 45 wt % di-pentaerythritol esters, and 23 to 50 wt % esters of tri and higher pentaerythritol oligomers for use in working fluids comprising fluorinated or highly fluorinated alkanes. In one example, an ISO 220 viscosity grade lubricant prepared from a mixture of C5 and C9 carboxylic acids wherein 78 mol % of the carboxylate groups are branched are shown to have good miscibility with tetrafluoroethane R-134a. However, the miscibility characteristics of fluorinated alkanes can differ greatly from the miscibility characteristics of fluorinated olefins, and as shown herein, polyolesters similar to those of U.S. Pat. No. 6,774,093 comprising C5 and branched C9 alkylcarboxylates demonstrate miscibility with the fluorinated olefin R-1234ze that is unsuitable for certain applications.
U.S. Published Patent Application No. 2010/0181523 discloses CO2 refrigeration working fluids comprising mono-pentaerythritol esters, di-pentaerythritol esters, esters of tri and higher pentaerythritol oligomers, wherein the alkyl carboxylate groups are linear alkyl carboxylates of 7 or more carbon atoms. U.S. Published Patent Application No. 2011/0240910 discloses CO2 refrigeration working fluids comprising mono-pentaerythritol esters, di-pentaerythritol esters, and esters of tri and higher pentaerythritol oligomers, wherein at least 50% of all alkyl carboxylate groups are linear alkyl carboxylates of 3 to 6 carbon atoms. While the CO2 refrigeration working fluids may also contain a halocarbon refrigerant, polyesters with a high percentage of linear carboxylate groups are not suitably miscible with the refrigerant when hydrofluoro-olefins are the predominate or sole refrigerant, as shown herein.
It has been found that high viscosity lubricants, i.e., lubricants with a kinematic viscosity at 40° C. of at least 150 cSt or higher, often 160 or higher, and in some cases 190 cSt, 200 cSt or higher, comprising certain mixtures of particular mono-carboxylic acids and pentaerythritol, di-pentaerythritol, and tri and higher pentaerythritol oligomers can be used with hydrofluoro-olefin refrigerants such as R-1234ze (1,3,3,3-tetrafluoropropene) to form high viscosity refrigeration working fluids without demonstrating the excessive refrigerant/lubricant solubility that can lead to an unacceptable decrease in viscosity, poor lubrication, wear foaming and capacity loss.